Broadband optical spectral shift analysis of dielectric coatings.

High-energy laser facilities require high reflection multilayer coatings on meter-scale substrates. Due to stringent use specifications, a precise control of deposition parameters is necessary to tailor the optical and mechanical properties of components. The resulting coatings are sensitive to relative humidity variations, leading to a shift of their optical spectra called spectral shift. This spectral shift is generally observed on a narrow range, near the operating wavelength. Here we extend the concept of spectral shift to a broader spectral range. This analysis serves as a tool to study the behavior of a multilayer coating spectrum with relative humidity. To validate the spectral shift determination method, we compared the spectral shift of single layers induced by the relative humidity with simulated optical properties induced by either thickness or refractive index variations. In addition to the validation of the approach, the fitting results and the comparison between spectral shift shapes show that relative humidity variations mainly impact the refractive index of the layers and SiO2 is more sensitive than HfO2.

Influence of absorption-edge properties on subpicosecond intrinsic laser-damage threshold at 1053 nm in hafnia and silica monolayers.

Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.

1.15 PW-850 J compressed beam demonstration using the PETAL facility.

The Petawatt Aquitaine Laser (PETAL) facility was designed and constructed by the French Commissariat à l'énergie atomique et aux énergies alternatives (CEA) as an additional PW beamline to the Laser MegaJoule (LMJ) facility. PETAL energy is limited to 1 kJ at the beginning due to the damage threshold of the final optics. In this paper, we present the commissioning of the PW PETAL beamline. The first kJ shots in the amplifier section with a large spectrum front end, the alignment of the synthetic aperture compression stage and the initial demonstration of the 1.15 PW @ 850 J operations in the compression stage are detailed. Issues encountered relating to damage to optics are also addressed.

Experimental demonstration of a synthetic aperture compression scheme for multi-Petawatt high-energy lasers.

We present the experimental demonstration of a subaperture compression scheme achieved in the PETAL (PETawatt Aquitaine Laser) facility. We evidence that by dividing the beam into small subapertures fitting the available grating size, the sub-beam can be individually compressed below 1 ps, synchronized below 50 fs and then coherently added thanks to a segmented mirror.

High reflection mirrors for pulse compression gratings.

We report an experimental investigation of high reflection mirrors used to fabricate gratings for pulse compression application at the wavelength of 1.053microm. Two kinds of mirrors are studied: the mixed Metal MultiLayer Dielectric (MMLD) mirrors which combine a gold metal layer with some e-beam evaporated dielectric bilayers on the top and the standard e-beam evaporated MultiLayer Dielectric (MLD) mirrors. Various samples were manufactured, damage tested at a pulse duration of 500fs. Damage sites were subsequently observed by means of Nomarski microscopy and white light interferometer microscopy. The comparison of the results evidences that if MMLD design can offer damage performances rather similar to MLD design, it also exhibits lower stresses; being thus an optimal mirror substrate for a pulse compression grating operating under vacuum.

Effect of electric field on laser induced damage threshold of multilayer dielectric gratings.

This paper studies gratings engraved in multilayer dielectric stacks for ultra high intensity laser compressors application. We design various grating profiles with high reflected efficiencies for 1780 l/mm multilayer dielectric gratings (MLD). Each grating is defined to exhibit a different electric field maximum value in the pillars of the grating. A damage testing facility operating at 1.053 mum, 500 fs pulse duration is used to damage test the parts manufactured from these designs. It is evidenced that for fixed incident angle and materials the damage of the grating is directly related to the electric field intensity maximum in the material, which depends on the groove profile. Laser induced damage thresholds of 5 J/ cm(2) are experimentally reached with very high and uniform efficiencies.